The present invention relates to a screen printing apparatus which prints paste, e.g., cream solder or conductive paste, onto a substrate, and a method of screen printing. More particularly, it relates to a novel squeegee head of the apparatus. The head can maintain fluidity of the paste and prevent defective printing due to hardened paste, and the method of screen printing using the same apparatus.
In an electronics-part-mounting process, a screen printing is employed for printing cream solder or conductive paste onto substrates. In this screen printing method, a mask platexe2x80x94having pattern apertures corresponding to the places to be printedxe2x80x94is placed over a substrate, and paste is printed onto the substrate through the pattern apertures with a squeegee head.
An airtight squeegee head has been used in a conventional squeegee method. This method is different from a regular screen printing, and it does not supply paste directly onto a mask plate, but employs a squeegee head in which paste is stored. With this method, a paste-contacting face provided beneath the squeegee head is brought into contact with a mask plate, and pressure is applied to the paste in the squeegee head, so that the paste is pushed into the pattern apertures of the mask plate via the paste-contacting face. Then the squeegee head is slid on the mask plate for filling each of the pattern apertures with the paste sequentially.
However, this screen printing using the conventional airtight squeegee head has had the following problems: In the airtight squeegee head, the paste is always applied pressure during the printing operation. The paste thus gradually loses fluidity and is hardened with the lapse of pressurizing time because granular component densely included in the paste is urged to each other. If such hardened paste is supplied onto the mask plate via the lower face of the squeegee head, the paste cannot be normally pushed into the pattern apertures. As a result, defective printings such as blur are produced. The conventional airtight squeegee head tends to incur defective printings due to the paste hardened by pressure.
The present invention addresses the problem discussed above and aims to provide a screen printing apparatus and a method of screen printing, both of which can prevent defective printings due to hardened paste.
The screen printing apparatus of the present invention slides a squeegee head on a mask plate, and prints paste on substrates via pattern apertures of a mask plate. The apparatus comprises a squeegee head, and the head includes the following elements:
(a) a paste storage for storing paste;
(b) a pressure applying member for applying pressure to the paste stored in the paste storage;
(c) a paste cell for accommodating pressurized paste and bringing the paste into contact with a surface of the mask plate via an opening formed on a lower face of the cell;
(d) a scraper for forming a brim of the opening of the cell by contacting with the surface of the mask plate at its lower end, and having slopes forming a front and a rear walls in a squeegee direction; and
(e) a paste-flow adjusting member for flowing the pressurized paste into the opening in a slant direction by blocking a flow into an area just above the opening, the adjusting member being disposed in the paste cell.
A screen printing method of the present invention prints the paste on substrates via pattern apertures of a mask plate by sliding a squeegee head on the mask plate, this method comprises the steps of:
(a) applying pressure to the paste in the storage disposed in the squeegee head;
(b) accommodating the paste in the paste cell having the opening at its lower face, and bringing the paste into contact with the surface of the mask plate;
(c) blocking the paste from flowing into an area just above the opening with the paste-flow adjusting member disposed in the paste cell, and flowing the paste into the opening in a slant direction.
The apparatus and the method discussed above allow the paste to maintain its fluidity, and prevent defective printings due to hardened paste.